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This scenario is actually mentioned in the "Fountain of Youth" trope, down in the description: "It's common for characters to overdo it in stories involving this trope, often winding up as babies, or possibly even regressing out of existence entirely."
Simply the Logical Extreme of Fountain of Youth.
Damn, guys, my bad! I think I kind of had a brain fart, I probably read that detail on the trope itself, then forgot where I got the idea and asked it here.
A character has access to a Fountain of Youth, but he overindulges. He gets younger, and younger, until he's a baby, then a fetus, then he pops out of existence.
Edited by Pisthetairos